Letters: Backwards signals

Rian Hughes draws some interesting points about data transmission, from his observation that a queue of stationary traffic that starts and stops at the head of the queue sends a signal backwards of these changes (Letters, 18 January). But I regret to say that he will not make his fortune with this technique.

A perfectly workable communications link can be made by sending a carrier signal from one end of a transmission line, end A, and modulating it by varying the impedance at the other end, B. The changes in impedance affect the line current, which is easily monitored at the first end. Thus data is sent from B to A. This is exactly what happens with the domestic electricity supply. When kettles are plugged in across the country to make tea during a commercial break, the event is clearly registered by the ammeters at the generating station.

A similar ...

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